CVE-2026-126075: medium-severity vulnerability in rails
Active Support: Possible XSS vulnerability in Active Support MessagePack serializer
Published
E-mail alert when it lands in KEV, gets a public PoC or its score goes up.
10Vexday Risk Score
No sign of exploitation. No public exploitation artifact known so far.
ssvc Trackcvss 4.7
Active Support is a collection of utility classes and standard library extensions used by Rails and other Ruby projects. From 8.1 until 8.1.4.1, ActiveSupport::MessagePack serializes an ActiveSupport::SafeBuffer as its bytes alone and deserializes it as a new SafeBuffer, so a buffer that is no longer html_safe? (for example, one mutated in place with gsub!) is restored as html_safe. If such a buffer holds untrusted input and is stored through a MessagePack serializer, rendering it after it is loaded bypasses ERB auto-escaping, possibly leading to XSS. Applications on Rails 8.1 are affected if they use a MessagePack serializer, such as config.action_dispatch.cookies_serializer = :message_pack or :message_pack_allow_marshal, or serializer: :message_pack for Rails.cache, ActiveSupport::MessageVerifier or ActiveSupport::MessageEncryptor, and store a SafeBuffer that was mutated in place with untrusted input. Rails 8.0 and earlier do not serialize SafeBuffer with MessagePack and are not affected. This issue is fixed in version 8.1.4.1.
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N
Affected products
rails · railsRelated CVEs — rails
In the same product, most dangerous first.
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